1. Field of the Invention
The present invention relates to a ceramic coating, and more particularly, to an oxidation resistant ceramic coating for carbon.
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.
2. Description of the Background Art
Oxidation protection coatings for carbon have been investigated and some protective coatings were developed. The oxidation protection coatings for carbon of the earlier art discloses carbides (predominantly SiC) that are typically used in the coatings because of good adherence to carbon. The following materials or combinations of materials are the primary components of the various coatings: silicon carbide, silicon nitride; silicon carbide, zirconium carbide; zirconium diboride, or hafnium diboride; and silicon carbide. All these coatings are applied by chemical vapor deposition or by melting on the surface of the carbon. Both of these application methods require high temperature heating in a protective atmosphere. Additionally, complex-shaped parts having intricate surface geometry and internal cavities are difficult to coat uniformly using these techniques.
The carbon substrate is heated at temperatures from 1000 to 1500xc2x0 C. (Celsius) for chemical vapor deposition, and from 1400 to 1850xc2x0 C. for melting silicon or zirconium on the surface of the carbon. Due to the high temperatures at which coatings are applied and differences in coefficients of thermal expansion (CTE) between the coating materials and the carbon, microcracks develop in the coatings during cooling to room temperature. The coatings have higher CTE than carbon. During use in an oxidizing atmosphere, the microcracks allow direct contact between oxygen and carbon, eliminating the oxidation protective capabilities of the coating. Additionally, the microcracks can lead to spalling of the coating. Traditional solutions to the microcracking problem are (1) to apply multiple layer coatings where the different materials in each layer minimize thermal expansion mismatch with the carbon, or (2) to introduce additives, such as Li2O or B2O3, which help to create low melting temperature glass which seals the microcracks. These approaches have yielded coatings which protect carbon from oxidation at temperatures below 1400xc2x0 C. for long term applications. At temperatures above 1500xc2x0 C., coatings provide oxidation protection for a few hours to a few minutes.
Exemplars of the art are U.S. Pat. No. 5,420,084 to Morel, issued on May 30, 1995, for Coatings for Protecting Materials Against Reactions with Atmosphere at High Temperatures, U.S. Pat. No. 5,759,688 to Lee et al., issued on Jun. 2, 1998, for Silicon Carbide Fiber Reinforced Carbon Composites, U.S. Pat. No. 6,221,942 B1 to Boakye et al., issued on Apr. 24, 2001, for Zircon-carbon for Ceramic Composite Fiber Coatings and Fine-grained Zircon Powder, U.S. Pat. No. 6,231,969 B1 to Knight et al., issued on May 15, 2001, for Corrosion, Oxidation And/or Wear-resistant Coatings, and U.S. Pat. No. 5,451,470 to Ashary et al., issued on Sep. 19, 1995, for Nickel-chromium Corrosion Coating and Process for Producing it.
It is therefore an object to provide a coating for carbon that is more resistant to oxidation.
It is another object to provide an oxidation resistant coating for carbon that can be cured at room temperature.
It is yet another object to provide an oxidation resistant coating for carbon that can be applied using conventional painting techniques on large or small intricately shaped structures.
It is yet another object to provide an oxidation resistant coating for carbon that reduces costs by lowering energy consumption and having no need for complex processing equipment.
To achieve the above and other objects of the present invention, there is provided a ceramic coating of zirconium diboride, silicon carbide, zirconium phosphate and silicon phosphate, which protects carbon-based materials from oxidation in high temperature oxidizing environments. The coating is applied at room temperature with brush, roller, squeegee, doctor blade, spray gun, etc., and cured at room temperature. The cured material forms a hard, protective ceramic shell. The coating can be applied to various carbon based materials including, but not limited to, amorphous carbon foam, graphitic foam, monolithic graphite, and carbon carbon composites.
Alternative compositions of the coating can be the partial or complete substitution of, hafnium diboride for zirconium diboride. Additional modifications of the coating can be accomplished by partial substitution of the borides or silicides of Ti, Ta, Cr, Nb; Ti, V, Re: for zirconium diboride. The present invention provides a composition for improved oxidation protection of carbon-based materials. The coating materials can be cured at room temperature and can be applied using conventional painting techniques on large or small intricately shaped structures. These features lead to significant decrease in cost because of lower energy consumption and no need for complex processing equipment.